The present invention relates generally to a sputtering system. More specifically, the invention relates to a sputtering system having an improved composition repeatability of a functional thin-film or the like, and an improved thin-film deposition rate thereof.
As a conventional sputtering system, there is widely used a parallel plate sputtering system shown in FIG. 1, i.e., a sputtering system wherein a substrate 1 faces a target 3 supported on a backing plate 2. There is also widely used a magnetron sputtering system wherein a magnet is provided in order to increase the thin-film deposition rate and reduce the thin-film deposited temperature. The thin-film deposition method using the parallel plate sputtering system is more simple than other thin-film deposition methods, such as vacuum evaporation, and able to control of various compositions of the thin film.
However, in the parallel plate sputtering system, particularly in a case where an oxide target or the like is used, negative ions, such as oxygen ions, emitted from the target 3 are accelerated by a cathode sheath to produce high energy particles, which irradiate the substrate 1 facing the target surface and damage the substrate 1. For that reason, there is a problem in that the deterioration of crystallinity, the divergence in composition, the roughness on the surface and so forth are caused on the thin-film formed on the substrate 1, so that it is difficult to form a high functional oxide film or the like.
In order to solve the aforementioned problems caused by the high energy particles, some attempts have been made when functional thin-films including oxide films, such as superconductive oxide films, ferromagnetic oxide films and ferroelectric oxide films, are formed by a sputtering method.
As a first method, there is known the off-axis alignment wherein the axis of the substrate 1 is perpendicular to the axis of the target 3 as shown in FIG. 2 (see, e.g., C. B. Eom et al, "In situ grown YBCO thin films from single-target magnetron sputtering", Appl. Phys. Lett. Vol.55, p595, 1989). In FIG. 2, reference number 4 denotes a substrate holder, 5 denotes a magnet, and 6 denotes a pole piece. The magnet 5 is associated with the pole piece 6 to produce lines M of magnetic force on the surface of the target 3.
As can be seen from FIG. 2, there is not much irradiation damage caused by particles radiated on the line extending from the target surface, since the substrate 1 is perpendicular to the target 3. However, in the off-axis sputtering system, there is another problem in that the uniformity on the surface of the substrate deteriorates. That is, in the off-axis alignment unlike the parallel plate sputtering system, the distance from the target 3 and the perspective angle are different at different positions on the substrate, so that the density, composition ratio and energy of incident particles are different at different positions. Therefore, there is a problem in that the differences in the film thickness distribution, composition and crystallinity are made between near and far sides from the target 3 on the substrate 1.
The above described ununiformity of inplane distribution is more conspicuous as the diameter of the substrate 1 increases. Although it appears that this problem can be solved by increasing the diameter of the target with respect to the diameter of the substrate, it is not possible to do so since the ununiformity of inplane distribution is under the strong control of the ratio of the mean free path of sputtered particles to the diameter of the substrate. That is, since a usual sputter gas pressure is in the range of from about 0.1 Pa to about 1 Pa, at which plasma is produced, the mean free path is in the range of from about 10 mm to about 100 mm.
The percentage of particles of high energy progressive components is large at a distance of the mean free path or less, and the percentage of low energy scattered particles is high as the distance increases. Therefore, it is very difficult to increase the diameter of the substrate in the off-axis aligned sputtering system.
As another example of an off-axis aligned sputtering system, there is known a system using two targets 3a and 3b facing each other as shown in FIG. 3 (see, e.g., Japanese Patent Laid-Open No. 57-158380). In this system, the two targets 3a and 3b facing each other are mounted on backing plates 2a and 2b, each of which has magnets 5 therein, and a substrate 1 is arranged on the side between the targets 3a and 3b so as to be turned by 90 degrees. All of the polarities of the magnets 5 in each of the targets 3a and 3b are the same, and the N pole of each of the magnets 5 faces the S pole of the adjacent magnet between the targets 3a and 3b. Therefore, a magnetic field is confined between the two targets 3a and 3b to produce a uniform magnetic field M perpendicular to the target surfaces.
Moreover, in order to increase the diameter of the substrate, there is proposed a sputtering system wherein a plurality of targets are arranged around a substrate in the form of a polygon (see Japanese Patent Laid-Open No. 6-17248). In this case, a uniform magnetic field is formed in a space surrounded by the plurality of targets.
If two or a plurality of targets are arranged around a substrate as described above, it is possible to improve the inplane uniformity. However, for the aforementioned reason, the diameter capable of ensuring the inplane uniformity is about twice as large as that when a single target is used, and it is only possible to ensure the inplane uniformity of a substrate having a diameter of about 100 mm. Therefore, it is very difficult to actually apply the off-axis aligned sputtering system even if the characteristic of the functional thin-film and so forth can be improved by the off-axis alignment, since it is required that an integrated circuit substrate should have a large diameter of 200 to 300 mm and strictly have inplane uniformity.
As a second method, there is known a method for using a cylindrical cathode 7, which has a magnet 5 therein and a cylindrical target 3 on the outer periphery thereof, to arrange substrates 1 around the cathode 7 as shown in FIGS. 4A and 4B. As a similar method, there is also known a method for using a hemispherical cathode to arrange substrates around the cathode so as to face each other via the cathode. If the cylindrical or hemispherical cathode is used, high energy particles are emitted radially, so that damage to the substrate 1 is less than that in the parallel plate sputtering system.
However, in the conventional sputtering systems using the cylindrical or hemispherical cathodes, the area of the target is small with respect to the area of the substrate, so that there is a problem in that the thin-film deposition rate decreases. Moreover, there is a problem in that it is difficult to prepare a cylindrical target having a large diameter, particularly it is very difficult to prepare a target of a ceramic, such as an oxide film. Therefore, it is difficult to cope with a large flat substrate, similar to the conventional off-axis sputtering systems.
As described above, according to the conventional typical off-axis sputtering systems, there is a problem in that the uniformity on the surface of a substrate deteriorates although irradiation damage caused by high energy particles reduces. Although the ununiformity on the surface of a substrate can be removed by arranging two or a plurality of targets around the substrate to produce a uniform magnetic field in a direction perpendicular to the surface of the target, this method can only ensure the inplane uniformity on a substrate having a diameter of about 100 mm or less, so that it is difficult to actually apply this method to an integrated circuit substrate which is required to have a large diameter of 200 to 300 mm.
On the other hand, the damage caused by high energy particles can be reduced by using the cylindrical or hemispherical cathode. However, in the conventional sputtering systems using the cylindrical or hemispherical cathodes, the thin-film deposition rate decreases, and it is very difficult to prepare a large cylindrical or hemispherical target of a ceramic, such as an oxide film, so that it is difficult to cope with a large flat substrate similar to the conventional off-axis sputtering systems.